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Managing Soil Health

Unit 1.1 | Part 1 – 25

SUPPLEMENT 1

A Soil Primer with Sustainable Management

Strategies

Soil is often thought of as an inert substrate, useful in propping up plants and a mere vehicle

for applied fertilizer and water. It is, in reality, a distinct ecosystem, defined as a system

formed by the interactions of a community of organisms with their physical environment.

Although we tend to take it for granted, human so-
ciety is principally possible only because the earth’s
crust is “dusted” with a thin and often fragile layer
of life-supporting material on which we can grow
food: the soil.
Two distinct parts of the soil—the biotic and
abiotic components—function together to form a
stable system. The biotic, or living (or that which
was once alive) component is comprised largely of
living plants, living organisms (macro and micro),
and organic matter (plant and animal residues),
which can be fresh, partially decomposed, or fully
stabilized (humus). Soil’s abiotic component is made
up of minerals, air, and water.
By understanding the soil’s ecosystem, growers
can harness and promote the biotic components
with judicious additions of compost and “green
manures” (cover crops worked into the soil) to
create a healthy environment for plant growth, and
thus virtually eliminate the need to apply purchased
fertilizer. This can lower costs—both out of pocket
and environmental.
This supplement introduces the soil’s four basic
components and three major properties, discusses
how these interact, and describes ways that garden-
ers and farmers can improve their soils by learning
how to become “biological growers.”

Soil’s Four Components

When you pick up a handful of soil, only half of
that volume is solid material (minerals and organic
matter). The other half should be pore space oc-
cupied by air (25%) and water (25%). Thus soil
consists of four basic components:

1. Mineral (45%, + or -, by volume): The mineral
   component of soil consists of rocks ground down
   over geologic time as a result of physical, chemi-
   cal and biological actions. Think of it as rock or
   stone “flour.”
   2. Organic matter (5%, + or -): Organic matter is
   made up of a wide range of organic (carbon-con-
   taining) substances, including living organisms,
   plant biomass, and the carbonaceous remains
   of organisms and plants. Some soil microorgan-
   isms break down the remains of plants, animals,
   and other microorganisms; others synthesize new
   substances.
   3. Soil air (25%): Soil air occupies the interstitial
   spaces between soil particles. Its primary role is
   to provide oxygen to fuel the aerobic (oxygen-
   requiring) activities of microorganisms and plant
   roots. Soil bacteria that associate with roots of
   legumes such a beans and peas use the nitrogen
   component of soil air to “fix” nitrogen in a form
   that plant roots can assimilate.
   4. Water (25%): Soil water or the soil solution
   carries dissolved nutrients that flow to and are
   actively intercepted by plant roots. Thus the soil
   solution is the vehicle for nutrients to “flow” into
   plants and, along with the products of photosyn-
   thesis, “grow” the plant. The soil solution also
   gives plants their turgor and rigidity.

Supplement 1: Soil Primer

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t FIGURE 1.1 | SOIL COMPOSITION:

AN IDEALIZED SOIL

ORGANIC MATTER 5%

MINERAL

45%

25% SOIL AIR

25% WATER